00:01
We will now proceed to follow the further development of the placentaand its relationship to the membranes that surround the developing fetus.
00:09
Once the placenta is in place,we have a situation where uterine arteries travel into placentabringing oxygen in blood to the embryo.
00:18
This maternal oxygen and blood fills intervillous spacesand allows gas exchange to occur across the villi.
00:26
Maternal blood interacts with deoxygenated bloodcoming from the umbilical arteries of the fetus,and even if though blood does not directly mingle,gas exchange and nutrients exchange occur across the villi.
00:39
Thereafter, umbilical veins take oxygenated blood back to the embryoand maternal veins take deoxygenated maternal blood from the intervillous spaceback into maternal circulation to be replenished.
00:54
Typically, the blood of the fetus and the mother should never directly connect.
01:00
One problem that arises with this situationis that as maternal blood enters the intervillous space,it necessarily mixes with blood that's already there;meaning that the oxygenated blood from the motheris mixing with the deoxygenated blood already present in the intervillous spacethat has not yet left through maternal vein.
01:19
What this means is that the oxygen content of the bloodin the intervillous space is lower than pure arterial blood.
01:26
To compensate for this, the fetus produces a fetal hemoglobinwhich binds to oxygen much more strongly than mature hemoglobin.
01:36
In physiology, we can say that the O2 P50 value of fetal hemoglobinis 19 millimeters of mercury,meaning it's about 50% saturated at a partial pressure of 19 millimeterscompared with 26.8 millimeters of mercury for adult hemoglobin.
01:55
Long story short, it binds to oxygen much more strongly than mature hemoglobinand allows the fetal hemoglobin to actually take the oxygenaway from the maternal circulationeven though we've got mixture of maternal oxygenatedand deoxygenated blood in the intervillous space.
02:14
Now, one problem that can also occur is that a fetal blood does contact maternal blooddue to a rupture of a villus or another problem,there can be immune reactions on the maternal blood to fetal blood.
02:28
In particular, if the fetal blood produces the D antigen and is therefore Rh positive,then it can create an immune reaction in a mother who is Rh negativeso she has not got that antigen on her own blood,she will produce antibodies against the D antigen and these IgG antibodiescan cross the placenta and attack fetal red blood cells.
02:54
Attacking the fetal red blood cells causes a condition known as erythroblastosis fetalis.
03:00
That's going to mean that we have an excessive creationof new red blood cells, erythroblasts,in the fetus to compensate for the red blood cellsthat are being attacked by the maternal antibodies.
03:12
This can progress to a condition called fetal hydrops or the tissues of the fetus swellin response to the immune attack from the mother's antibodies,and in less severe but still serious forms you can have breakdown of fetal blood cellsproducing excessive bilirubin leading to jaundice as development proceeds further.
03:31
This response becomes more and more severe for subsequent pregnanciesbecause the maternal immune system is already primed to produce those antibodiesagainst the D antigen in Rh positive blood.
03:44
The placenta, initially, is going to form as a spherical objectsurrounding the developing embryo in the uterine lining,but as the embryo and placenta enlargeit starts to push its way into the cavity of the uterus.
03:57
As this occurs, the placenta thins on one side and will eventually formmore or less a pancake shape structure on its attachment to the uterusand not be present outside the fetus beyond that.
04:10
As the embryo enlarges, the chorionic cavity which contains the yolk sac will thinand the amniotic cavity, shown here in blue, will expand outward tremendouslygiving buoyancy to the fetus and supporting it as it moves but also helping it resist gravity.
04:28
The portion of the endometrium, the lining of the uterus that's pushed outwardas the embryo grows on the uterine wall is gonna be known as the decidua capsularisand the lining of the endometrium everywhere else is called the decidua parietalis.
04:45
At the point where the decidua parietalis of the uterine wallmeets the decidua capsularis covering the developing fetus,we have an area called the decidua basalisand that's just marking the subdivisionsof the endometrial lining as the fetus continues to develop.
05:02
As it develops, the fetus becomes larger and larger.
05:05
The placenta is more or less pushed to one side, and as I said before,from the relatively pancake-shaped structure,most likely on the posterior wall of the uterus.
05:18
It's connected to the developing fetus by the umbilical cord,which is full of a loose mesenchymecalled Wharton's jelly that surrounds the vesselsthat are travelling in the umbilical cord to and from the placenta.
05:31
The membrane that supports the fetus consist of the amniotic membraneand then a smooth layer of the chorion, a very thin layer of the endometriumthat was present when the fetus grew out of the uterine walland then another smooth layer of the chorion.
05:49
This amniochorionic membrane is what ruptures when delivery is imminentand is known as the water breakingthat allows the amniotic fluid to exit the birth canaland set the stage for a hopefully smooth delivery.
06:03
By the 14th or 15th week of pregnancy,there's enough amniotic fluid surrounding the developing fetusthat we can safely sample some of it via amniocentesis.
06:14
This is generally done in order to check some genetic issues that may be arising in the fetusand looking for chromosomal abnormalities,markers of neural tube defects such as alpha-fetoproteinand other possible issues that might be anticipated.
06:28
Alternatively, the chorionic villi can be sampled and then taken for a genetic analysis.
06:35
This can be done either through the anterior body wall using ultrasound or vaginally,to harvest some of the chorionic villi.

About the Lecture

The lecture Placenta and Fetal Membranes by Peter Ward, PhD is from the course Conception, Implantation and Fetal Development.

Author of lecture Placenta and Fetal Membranes

Peter Ward, PhD

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